Makes me think, Discord is thousands of years old, right? An Equestrian Girls counterpart shouldn't exist unless that version of him was also a supernatural/mystical entity, since if he was just a person he'd have lived in a distant age and died long before Equestria Girls. I kinda hope he is a mystical entity when/if he appears though. My idea for him is that he's a Chaos Spirit who has currently taken the guise of an old curator of a Museum which itself has mystical properties, like, the exhibits can come to life or something.

Discord wasn’t really a bad guy, if anything more like a prankster. He is the embodiment of chaos. He wasn’t a after the throne, he just wanted to screw with ponies by literally fliping everything around. Discord never had friends and such never had to take that into acount of his actions. Seeing Fluttershy turn away from him after breaking his promess of not causing a colamety finally made him see his actions do have a price, and he didn’t want to lose the one friend he’s ever had. This is my thought anyways.